Looking at some of the other templates, it seems common to use a calendar-color class to indicate a background color/styling, so at first look, it seems more uniform to stick with that approach here, and could allow in the future, if it was useful (currently I don't think it is) to allow for an accessibility string that is invisible but would show up for screen readers.

So I did not do a change for this yet, but if you feel the benefits of the icon font outweigh the above, I can do the change.